Unidentified X-ray Sources Research Articles

Multiwavelength identification of millisecond pulsar candidates in the Galactic bulge

Context. The existence of a population of millisecond pulsars in the Galactic bulge is supported, along with other evidence, by the Fermi GeV excess, an anomalous γ-ray emission detected almost 15 years ago in the direction of the Galactic center. However, radio surveys searching for pulsations have not yet revealed bulge millisecond pulsars. Aims. Identifying promising bulge millisecond pulsar candidates is key to motivating pointed radio pulsation searches. Candidates are often selected among steep-spectrum or polarized radio sources, but multiwavelength information can also be exploited: The aim of this work is to pinpoint strong candidates among the yet unidentified X-ray sources. Methods. We investigated the multiwavelength counterparts of sources detected by the Chandra X-ray observatory that have spectral properties expected for millisecond pulsars in the Galactic bulge. We considered that ultraviolet, optical, and strong infrared counterparts indicate that an X-ray source is not a bulge pulsar, while a radio or a faint infrared counterpart makes it a promising candidate. Results. We identify a large population of more than a thousand X-ray sources without optical, ultraviolet, or strong infrared counterparts. Among them, five are seen for the first time in unpublished radio imaging data from the Very Large Array. We provide the list of promising candidates, for most of which follow-up pulsation searches are ongoing.

Dataset of Classified Chandra Sources in Globular Clusters

We present a collection of classified X-ray sources in Globular Clusters (GCs) observed by the Chandra X-ray Observatory, including active binaries, cataclysmic variables, millisecond pulsars, and low-mass X-ray binaries. We cross-match the most accurate published positions from multiwavelength observations of these sources to the Chandra Source Catalog Release 2.1, and the HST UV Globular Cluster Survey to extract their multiwavelength properties. The data set can be accessed via an interactive website (https://home.gwu.edu/~kargaltsev/XCLASS_GC) and used as a training data set for machine-learning classification of unidentified X-ray sources in GCs.

XTE J1906+090: a persistent low-luminosity Be X-ray binary

ABSTRACT We present new results from INTEGRAL and Swift observations of the hitherto poorly studied and unidentified X-ray source XTE J1906+090. A bright hard X-ray outburst (luminosity of ∼1036 erg s−1 above 20 keV) has been discovered with INTEGRAL observations in 2010, this being the fourth outburst ever detected from the source. Such events are sporadic, the source duty cycle is in the range (0.8–1.6) per cent as inferred from extensive INTEGRAL and Swift monitoring in a similar hard X-ray band. Using five archival unpublished Swift/X-Ray Telescope (XRT) observations, we found that XTE J1906+090 has been consistently detected at a persistent low X-ray luminosity value of ∼1034 erg s−1, with limited variability (a factor as high as 4). Based on our findings, we propose that XTE J1906+090 belongs to the small and rare group of persistent low-luminosity Be X-ray binaries.

Classifying Unidentified X-Ray Sources in the Chandra Source Catalog Using a Multiwavelength Machine-learning Approach

The rapid increase in serendipitous X-ray source detections requires the development of novel approaches to efficiently explore the nature of X-ray sources. If even a fraction of these sources could be reliably classified, it would enable population studies for various astrophysical source types on a much larger scale than currently possible. Classification of large numbers of sources from multiple classes characterized by multiple properties (features) must be done automatically and supervised machine learning (ML) seems to provide the only feasible approach. We perform classification of Chandra Source Catalog version 2.0 (CSCv2) sources to explore the potential of the ML approach and identify various biases, limitations, and bottlenecks that present themselves in these kinds of studies. We establish the framework and present a flexible and expandable Python pipeline, which can be used and improved by others. We also release the training data set of 2941 X-ray sources with confidently established classes. In addition to providing probabilistic classifications of 66,369 CSCv2 sources (21% of the entire CSCv2 catalog), we perform several narrower-focused case studies (high-mass X-ray binary candidates and X-ray sources within the extent of the H.E.S.S. TeV sources) to demonstrate some possible applications of our ML approach. We also discuss future possible modifications of the presented pipeline, which are expected to lead to substantial improvements in classification confidences.

Searching for Cataclysmic Variable Stars in Unidentified X-Ray Sources

We carry out a photometric search for new cataclysmic variable stars (CVs), with the goal of identification for candidates of AR Scorpii-type binary systems. We select GAIA sources that are likely associated with unidentified X-ray sources, and analyze the light curves taken by the Zwicky Transient Facility, Transiting Exoplanet Survey Satellite, and Lulin One-meter Telescope in Taiwan. We investigate eight sources as candidates for CVs, among which six sources are new identifications. Another two sources have been recognized as CVs in previous studies, but no detailed investigations have been done. We identify two eclipsing systems that are associated with an unidentified XMM-Newton or Swift source, and one promising candidate for polar associated with an unidentified ASKA source. Two polar candidates may locate in the so-called period gap of a CV, and the other six candidates have an orbital period shorter than that of the period gap. Although we do not identify a promising candidate for AR Scorpii-type binary systems, our study suggests that CV systems that have X-ray emission and do not show frequent outbursts may have been missed in previous surveys.

X-ray investigation of the X-ray source 1RXS J121902.7–630910 with Suzaku

We present a study of the unidentified X-ray source 1RXS J121902.7–630910 based on Suzaku X-ray Imaging Spectrometer observation. We aim at clarifying the nature of the emission from 1RXS J121902.7–630910. We found that the spectrum is well represented by a thermal plasma model with an electron temperature of kTe∼ 3.7 keV, the upper limit of ionization time-scale of τ∼ 5.6 ×1010 cm−3 s, and the absorbing column density of NH∼ 0.5 ×1021 cm−2. We investigated the possible origin of 1RXS J121902.7–630910, and found optical counterpart candidates to this X-ray source. We conclude that it could be a point-like source compared to the analysis results of an extended source, and observations at other wavelengths are needed to confirm its nature.

SRG/ART-XC all-sky X-ray survey: Catalog of sources detected during the first year

We present a first catalog of sources detected by theMikhail PavlinskyART-XC telescope on board the SRG observatory in the 4–12 keV energy band during its ongoing all-sky survey. The catalog comprises 867 sources detected on the combined map of the first two 6-month scans of the sky (December 2019–December 2020), ART-XC sky surveys 1 and 2, or ARTSS12. The achieved sensitivity to point sources varies between ~4 x 10-12erg s-1cm-2near the ecliptic plane and ~8 x 10-13erg s-1cm-2(4–12keV) near the ecliptic poles, and the typical localization accuracy is ~15”. Of the 750 sources of known or suspected origin in the catalog, 56% are extragalactic (mostly active galactic nuclei, AGN; and clusters of galaxies) and the rest are Galactic (mostly cataclysmic variables, CVs; and low- and high-mass X-ray binaries). For 114 sources, ART-XC has detected X-rays for the first time. Although the majority of these (~80) are expected to be spurious (given the adopted detection threshold), there can be a significant number of newly discovered astrophysical objects. We have started a program of optical follow-up observations of the new and previously unidentified X-ray sources, which has already led to the identification of several AGN and CVs. With the SRG all-sky survey planned to continue for a total of four years, we can expect the ART-XC survey in the 4–12 keV band to significantly surpass previous surveys that were carried out in similar (medium X-ray) energy bands in terms of the combination of angular resolution, sensitivity, and sky coverage.

NuSTAR Detection of a Hard X-Ray Source in the Supernova Remnant-molecular Cloud Interaction Site of IC 443

Abstract We report on a broadband study of a complex X-ray source (1SAX J0618.0+2227) associated with the interaction site of the supernova remnant (SNR) IC 443 and ambient molecular cloud (MC) using NuSTAR, XMM-Newton, and Chandra observations. Its X-ray spectrum is composed of both thermal and nonthermal components. The thermal component can be equally well represented by either a thin plasma model with kT = 0.19 keV or a blackbody model with kT = 0.11 keV. The nonthermal component can be fit with either a power law with Γ ∼ 1.7 or a cutoff power law with Γ ∼ 1.5 and a cutoff energy at E cut ∼ 18 keV. Using the newly obtained NuSTAR data set, we test three possible scenarios for isolated X-ray sources in the SNR–MC interaction site: (1) a pulsar wind nebula (PWN); (2) an SNR ejecta fragment; and (3) a shocked molecular clump. We conclude that this source is most likely composed of an SNR ejecta (or a PWN) and surrounding shocked molecular clumps. The nature of this hard X-ray source in the SNR–MC interaction site of IC 443 may shed light on unidentified X-ray sources with hard X-ray spectra in rich environments for star-forming regions, such as the Galactic center.

AChandraX-ray study of the young star cluster NGC 6231: low-mass population and initial mass function

NGC6231 is a massive young star cluster, near the center of the Sco OB1 association. While its OB members are well studied, its low-mass population has received little attention. We present high-spatial resolution Chandra ACIS-I X-ray data, where we detect 1613 point X-ray sources. Our main aim is to clarify global properties of NGC6231 down to low masses through a detailed membership assessment, and to study the cluster stars' spatial distribution, the origin of their X-ray emission, the cluster age and formation history, and initial mass function. We use X-ray data, complemented by optical/IR data, to establish cluster membership. The spatial distribution of different stellar subgroups also provides highly significant constraints on cluster membership, as does the distribution of X-ray hardness. We perform spectral modeling of group-stacked X-ray source spectra. We find a large cluster population down to ~0.3 Msun (complete to ~1 Msun), with minimal non-member contamination, with a definite age spread (1-8 Myrs) for the low-mass PMS stars. We argue that low-mass cluster stars also constitute the majority of the few hundreds unidentified X-ray sources. We find mass segregation for the most massive stars. The fraction of circumstellar-disk bearing members is found to be ~5%. Photoevaporation of disks under the action of massive stars is suggested by the spatial distribution of the IR-excess stars. We also find strong Halpha emission in 9% of cluster PMS stars. The dependence of X-ray properties on mass, stellar structure, and age agrees with extrapolations based on other young clusters. The cluster initial mass function, computed over ~2 dex in mass, has a slope Gamma~-1.14. The total mass of cluster members above 1 Msun is 2280 Msun, and the inferred total mass is 4380 Msun. We also study the peculiar, hard X-ray spectrum of the Wolf-Rayet star WR79.

Discovery of a deep Seyfert-2 galaxy atz = 0.222 behind NGC 300

We report on the unveiling of the nature of the unidentified X-ray source 3XMM J005450.3-373849 as a Seyfert-2 galaxy located behind the spiral galaxy NGC 300 using Hubble Space Telescope data, new spectroscopic Gemini observations and available XMM-Newton and Chandra data. We show that the X-ray source is positionally coincident with an extended optical source, composed by a marginally resolved nucleus/bulge, surrounded by an elliptical disc-like feature and two symmetrical outer rings. The optical spectrum is typical of a Seyfert-2 galaxy redshifted to z=0.222 +/- 0.001, which confirms that the source is not physically related to NGC 300. At this redshift the source would be located at 909+/-4 Mpc (comoving distance in the standard model). The X-ray spectra of the source are well-fitted by an absorbed power-law model. By tying $N_\mathrm{H}$ between the six available spectra, we found a variable index $\Gamma$ running from ~2 in 2000-2001 years, to 1.4-1.6 in the 2005-2014 period. Alternatively, by tying $\Gamma$, we found variable absorption columns of N_H ~ 0.34 x $10^{-22}$ cm$^{-2}$ in 2000-2001 years, and 0.54-0.75 x $10^{-22}$ cm$^{-2}$ in the 2005-2014 period. Although we cannot distinguish between an spectral or absorption origin, from the derived unabsorbed X-ray fluxes, we are able to assure the presence of long-term X-ray variability. Furthermore, the unabsorbed X-ray luminosities of 0.8-2 x 10$^{43}$ erg s$^{-1}$ derived in the X-ray band are in agreement with a weakly obscured Seyfert-2 AGN at $z \approx 0.22$.

XMM-Newton and Suzaku spectroscopic studies of unidentified X-ray sources towards the Galactic bulge: 1RXS J180556.1−343818 and 1RXS J173905.2−392615

Abstract With XMM-Newton and Suzaku observations, for the first time, we acquired broad-band spectra of two unidentified X-ray sources towards the Galactic bulge: 1RXS J180556.1−343818 and 1RXS J173905.2−392615. The 1RXS J180556.1−343818 spectrum in the 0.3–7 keV band was explained by X-ray emission that originated from an optically-thin thermal plasma with temperatures of 0.5 and 1.8 keV. The estimated absorption column density of NH ∼ 4 × 1020 cm−2 was significantly smaller than the Galactic H i column density towards the source. A candidate for its optical counterpart, HD 321269, was found within 4″. In terms of the X-ray properties and the positional coincidence, it is quite conceivable that 1RXS J180556.1−343818 is an active G giant. We also found a dim X-ray source that was positionally consistent with 1RXS J173905.2−392615. Assuming that the X-ray spectrum can be reproduced with an absorbed, optically-thin thermal plasma model with kT = 1.6 keV, the X-ray flux in the 0.5–8 keV band was 8.7 × 10−14 erg s−1 cm−2, fainter by a factor of ∼7 than that of 1RXS J173905.2−392615 during the ROSAT observation. The follow-up observations we conducted revealed that these two sources would belong to the Galactic disk, rather than the Galactic bulge.

On the origin of two unidentified radio/X-ray sources discovered withXMM-Newton

We aim at clarifying the nature of the emission of two spatially related unidentified X-ray sources detected with XMM-Newton telescope at intermediate-low Galactic latitude. Observations reveal a point-like source aligned with elongated diffuse emission. The X-ray spectra are best-fitted by absorbed power laws with photon indices ~1.7 for the point-like and ~2.0 for the extended one. Both sources show nonthermal radio-continuum counterparts that might indicate a physical association. From the available data, we did not detect variability on the point-like source in several timescales. Two possible scenarios are analyzed: first, based on HI line absorption, assuming a Galactic origin, we infer a distance upper bound of <2 kpc, which poses a constraint on the height over the Galactic plane of <200 pc and on the linear size of the system of <2.3 pc. In this case, the X-ray luminosities are >10^32 erg/s and >7.5 x 10^32 erg/s, for the point-like and extended sources, respectively; second, an extra-Galactic nature is discussed, where the point-like source might be the core of a radio galaxy and the extended source its lobe. In this case, we compare derived fluxes, spectral indices, and spatial correlation with those typical from the radio galaxy population, showing the feasibility of this alternative astrophysical scenario. From the available observational evidence, we suggest that the most promising scenario to explain the nature of these sources is a system consisting of a one-sided radio galaxy, where the point-like source is an active galactic nucleus and the extended source corresponds to the emission from its lobe. Other possibilities include a PSR/PWN origin, where the radio/X-ray emission originates from the synchrotron cooling of relativistic particles in the PSR magnetic field or a casual alignment between two unrelated sources, such as an AGN core and a Galactic X-ray blob.

A New Cluster of Galaxies Towards the Galactic Bulge, Suzaku J1759–3450

Abstract We observed an extended X-ray source, designated as Suzaku J1759–3450, with Suzaku and Chandra observations towards 1RXS J175911.0–344921, which is an unidentified X-ray source listed in the ROSAT Bright Source Catalogue. A conspicuous emission line at 6 keV was also found in the Suzaku J1759–3450 spectrum. Assuming the emission line to be K emission from He-like Fe ions, we inferred Suzaku J1759–3450 to be an extragalactic object located at z = 0.13. The radial profile of the surface brightness in the 0.5–10 keV band was explained well with an isothermal β-model of $r_\textrm c=1{.^\prime}61$ and $\beta=0.78$. The X-ray spectrum was well reproduced by an opticallythin thermal plasma with the electron temperature of $kT_e = 5.3$ keV, attenuated by the photoelectric absorption of $N_\textrm H = 2.3 \times 10^{21} \textrm c\textrm m^{-2}$. The bolometric X-ray luminosity of $L_\textrm X(r &amp;lt; r_{500}) = 4.3 \times 10^{44} \textrm {erg} \textrm s^{-1}$ is consistent with that expected from the LX–T relation of clusters of galaxies. In terms of the spatial extent, the X-ray spectrum, and the bolometric luminosity of the X-ray emitting gas, we concluded that Suzaku J1759–3450 is a new cluster of galaxies.

BL LAC CANDIDATES FOR TeV OBSERVATIONS

BL Lac objects are the most numerous class of extragalactic TeV-detected sources. One of the biggest difficulties in investigating their TeV emission resides in their limited number, since only 47 BL Lacs are known as TeV emitters. In this paper, we propose new criteria to select TeV BL Lac candidates based on the infrared (IR) and X-ray observations. We apply our selection criteria to the BL Lac objects listed in the ROMA-BZCAT catalog so identifying 41 potential TeV emitters. We then consider a search over a more extended sample combining the ROSAT bright source catalog and the WISE all-sky survey revealing 54 additional candidates for TeV observations. Our investigation also led to a tentative classification of 16 unidentified X-ray sources as BL Lac candidates. This analysis provides new interesting BL Lac targets for future observations with ground based Cherenkov telescopes.

Suzaku Observations of Unidentified X-Ray Sources toward the Galactic Bulge

Abstract We present Suzaku observations of two unidentified sources towards the Galactic bulge, 1RXS J165256.3$-$264503 and 1RXS J174459.5$-$172640, both of which were discovered with the ROSAT All-Sky Survey. We obtained wide-band spectra of these sources above 2 keV for the first time. The X-ray spectrum of 1RXS J165256.3$-$264503 consists of two absorbed thin thermal plasma components with temperatures of 0.7 and 2.4 keV. An RS CVn star, HD 152178, was found within a 90% error circle of the source position. Due to this position coincidence, and an X-ray spectrum analogous to those of other RS CVn binaries, we concluded that 1RXS J165256.3$-$264503 is an RS CVn-type active binary. The X-ray spectrum of 1RXS J174459.5$-$172640 was dominated by continuum emission extending up to $\sim$ 20 keV, which can be explained by an absorbed cutoff power-law or a combination of absorbed multicolor disk-blackbody and Comptonized emission. We inferred that the source is a new Galactic-bulge low-mass X-ray binary (LMXB) with a luminosity of 2 $\times$ 10$^{35}$ erg s$^{-1}$, based on its X-ray spectrum and lightcurve. Hence, 1RXS J174459.5$-$172640 may be an intriguing example of an LMXB in an intermediate state in which X-ray emission mechanisms can be readily explored.

THE DEEP LOOK AT THE HARD X-RAY SKY: THESWIFT–INTEGRALX-RAY (SIX) SURVEY

The super-massive black-holes in the centers of Active Galactic Nuclei (AGNs) are surrounded by obscuring matter that can block the nuclear radiation. Depending on the amount of blocked radiation, the flux from the AGN can be too faint to be detected by currently flying hard X-ray (above 15 keV) missions. At these energies only ~1% of the intensity of the Cosmic X-ray Background (CXB) can be resolved into point-like sources that are AGNs. In this work we address the question of the undetected sources contributing to the CXB with a very sensitive and new hard X-ray survey: the SIX survey that is obtained with the new approach of combining the Swift/BAT and INTEGRAL/IBIS X-ray observations. We merge the observations of both missions. This enhances the exposure time and reduces systematic uncertainties. As a result we obtain a new survey over a wide sky area of 6200 deg^2 that is more sensitive than the surveys of Swift/BAT or INTEGRAL/IBIS alone. Our sample comprises 113 sources: 86 AGNs (Seyfert-like and blazars), 5 galaxies, 2 clusters of galaxies, 3 Galactic sources, 3 previously detected unidentified X-ray sources, and 14 unidentified sources. The scientific outcome from the study of the sample has been properly addressed to study the evolution of AGNs at redshift below 0.4. We do not find any evolution using the 1/V_max method. Our sample of faint sources are suitable targets for the new generation hard X-ray telescopes with focusing techniques.

IGR J17354−3255 as a candidate intermediate supergiant fast X-ray transient possibly associated with the transient MeV AGL J1734−3310

We present spectral and temporal results from INTEGRAL long-term monitoring of the unidentified X-ray source IGR J17354-3255. We show that it is a weak persistent hard X-ray source spending a major fraction of the time in an out-of-outburst state with average 18-60 keV X-ray flux of about 1.1 mCrab, occasionally interspersed with fast X-ray flares (duration from a few hours to a few days) with a dynamic range as high as 200. From archival Swift/XRT observations, we also show that the dynamic range from non-detection to highest level of measured X-ray activity is >300. Our IBIS timing analysis strongly confirms the 8.4 days orbital period previously detected with Swift/BAT, in addition we show that the shape of the orbital profile is rather smooth and appears to be dominated by low level X-ray emission rather than by bright outbursts, the measured degree of outburst recurrence is about 25 per cent. The spectral and temporal characteristics of IGR J17354-3255 are highly indicative of a Supergiant High Mass X-ray Binary nature (SGXB). However, our inferred dynamic ranges both at soft and hard X-rays are significantly greater than those of classical SGXB systems, but instead are typical of intermediate Supergiant Fast X-ray Transient (SFXTs). Finally, we note for the first time that the observed fast flaring X-ray behaviour of IGR J17354-3255 is very similar to that detected with AGILE from the spatially associated MeV source AGL J1734-3310, suggesting a possible physical link between the two objects.

Identification of new cataclysmic variables in the 1RXS and USNO-B1.0 catalogs

As a result of applying the original optical variability search method on publicly available data, we have found eight new cataclysmic variables and two possible Optically Violent Variable quasars among the previously unidentified X-ray sources in the ROSAT catalog. We describe the search method and present the characteristics of the newly identified variable stars. The obtained results demonstrate the large potential of the concept of Virtual Observatory for identifying new objects of astrophysical interest.

IDENTIFICATION OF A POPULATION OF X-RAY-EMITTING MASSIVE STARS IN THE GALACTIC PLANE

We present X-ray, infrared, optical and radio observations of four previously unidentified Galactic plane X-ray sources, AX J163252-4746, AX J184738-0156, AX J144701-5919 and AX J144547-5931. Detection of each source with the Chandra X-ray Observatory has provided sub-arcsecond localizations, which we use to identify bright infrared counterparts to all four objects. Infrared and optical spectroscopy of these counterparts demonstrate that all four X-ray sources are extremely massive stars, with spectral classifications Ofpe/WN9 (AX J163252-4746), WN7 (AX J184738-0156 = WR121a), WN7-8h (AX J144701-5919) and OIf+ (AX J144547-5931). AX J163252-4746 and AX J184738-0156 are both luminous, hard, X-ray emitters with strong Fe XXV emission lines in their X-ray spectra at ~6.7 keV. The multi-wavelength properties of AX J163252-4746 and AX J184738-0156 are not consistent with isolated massive stars or accretion onto a compact companion; we conclude that their X-ray emission is most likely generated in a colliding-wind binary system. For both AX J144701-5919 and AX J144547-5931, the X-ray emission is an order of magnitude less luminous and with a softer spectrum. These properties are consistent with a colliding-wind binary interpretation for these two sources also, but other mechanisms for the generation of X-rays cannot be excluded. There are many other as yet unidentified X-ray sources in the Galactic plane, with X-ray properties similar to those seen for AX J163252-4746, AX J184738-0156, AX J144701-5919 and AX J144547-5931. This may indicate a substantial population of X-ray-emitting massive stars and colliding-wind binaries in the Milky Way.

Swift follow-up of unidentified X-ray sources in the XMM-Newton Slew Survey

We present deep Swift follow-up observations of a sample of 94 unidentified X-ray sources from the XMM–Newton Slew Survey. The X-ray Telescope (XRT) on-board Swift detected 29 per cent of the sample sources; the flux limits for undetected sources suggest the bulk of the Slew Survey sources are drawn from one or more transient populations. We report revised X-ray positions for the XRT-detected sources, with typical uncertainties of 2.9 arcsec, reducing the number of catalogued optical matches to just a single source in most cases. We characterize the sources detected by Swift through their X-ray spectra and variability and via Ultraviolet–Optical Telescope photometry and using catalogued near-infrared, optical and radio observations of potential counterparts. Six sources can be associated with known objects and eight sources may be associated with unidentified ROSAT sources within the 3σ error radii of our revised X-ray positions. We find 10 of the 30 XRT- and/or Burst Alert Telescope (BAT)-detected sources are clearly stellar in nature, including one periodic variable star and two high proper motion stars. For 11 sources we propose an active galactic nucleus (AGN) classification, among which four are detected in hard X-rays and three have redshifts spanning z= 0.2–0.9 obtained from the literature or from optical spectroscopy presented here. A further three sources are suspected AGN and one is a candidate Galactic hard X-ray flash, while five sources remain unclassified. The 67 Slew Survey sources we do not detect with Swift XRT or BAT are studied via their characteristics in the Slew Survey observations and by comparison with the XRT- and BAT-detected population. We suggest that these are mostly if not all extragalactic, though unlikely to be highly absorbed sources in the X-rays such as Compton thick AGN. A large number of these are highly variable soft X-ray (0.2–2 keV) sources and a smaller number are highly variable hard (2–12 keV) sources. A small fraction of mainly hard-band Slew Survey detections may be spurious. This follow-up programme brings us a step further to completing the identifications of a substantial sample of XMM–Newton Slew Survey sources, important for understanding the nature of the transient sky and allowing flux-limited samples to be constructed.